1. Field of the Invention
This invention relates to a mode-locked laser apparatus that generates an ultra-high-speed light pulse train with a stable repetition rate such as that required for high-capacity communication systems and the like, and particularly to a mode-locked laser apparatus that uses the chromatic dispersion characteristics of the optical path to generate a feedback signal and adjusts the length of the optical path using this signal.
2. Description of the Prior Art
Pulse generators having a high repetition rate and that are optical sources with a uniform repetition rate that can be synchronized to an external clock signal are important in the field of optical communication. Recently, by adapting mode-locking techniques to fiber lasers, research into generating such high-repetition pulse trains has become active.
A known method of generating pulses by mode locking is the method of installing an intensity modulator or phase modulator in a ring laser oscillator and modulating the intensity or phase of the light passing through the modulator. At this time, the modulation frequency fm required to achieve the optimal mode locking can be expressed by the following Equation 1.                               f          m                =                              N            ·                          (                              c                                  n                  ⁢                                      xe2x80x83                                    ⁢                  L                                            )                                =                      N            ·                          f              r                                                          (        1        )            
Here, c is the speed of light, n is the index of refraction of the optical fiber, L is the length of the optical path of the oscillator, fr (=c/nL) is the fundamental repetition rate of the laser, and N is a positive integer. When fm and fr have the aforementioned relationship, a periodic light pulse is generated from the laser and the repetition rate of the pulse becomes the same as the modulation frequency fm. While one pulse is present within the oscillator in the case of N=1, N pulses are present at equal intervals in the case of N greater than 1. Typically, fr is between several hundred kHz and several dozen MHz. For this reason, in order to generate a pulse train with a repetition rate in the GHz band required for optical communications, mode locking is performed by modulation in the state N greater than  greater than 1. Mode locking in the case of N greater than 1 in this manner is typically called harmonic mode locking.
In order to increase the repetition rate of mode-locked pulses, from the above explanation it can be seen that it is sufficient to increase the modulation frequency. However, the modulation frequency has an upper limit given by the bandwidth of the modulator or the radiofrequency (RF) oscillator that generates the modulation signal. In passing, optical modulators with a frequency bandwidth of 40 GHz have recently become commercially available and there are reports of them being used to generate 40-GHz mode-locked pulses by mode locking.
In addition, higher-order mode locking methods that exceed the bandwidth of the modulator and RF oscillator and the like in order to increase the repetition rate of the optical pulses have been proposed in Reference 1 (K. S. Abedin, N. Onodera and M. Hyodo, xe2x80x9cRepetition-rate multiplication in actively mode-locked fiber lasers by higher-order FM mode locking using a high-finesse Fabry-Perot filter,xe2x80x9d Applied Physics Letters, Vol. 73, No. 10, pp. 1311-1313, 1998), Reference 2 (K. S. Abedin, N. Onodera and M. Hyodo, xe2x80x9cOvercoming the repetition-rate-multiplication imposed by free-spectral-range of the Fabry-Perot filter used in higher-order FM mode-locked lasers,xe2x80x9d Electronics Letters, Vol. 34, No. 23, pp. 2264-2265) and Reference 3 (K. S. Abedin et al., xe2x80x9cGeneration of a 64-GHz, 3.3-ps transform-limited pulse train from a fiber laser employing higher-order frequency-modulated mode locking,xe2x80x9d Optics Letters, Vol. 24, No. 22, pp. 1564-1566 (1999).) and the like. By means of any one of the above methods, a pulse train with a repetition rate of an integral multiple of the modulation frequency can be generated from a mode-locked laser, so the realization of a higher-order mode-locked laser pulse generator that exceeds the bandwidth limit due to the modulator as above became possible.
FIG. 1 shows one example of such a higher-order mode-locked laser pulse generator. The apparatus shown in FIG. 1 consists primarily of an optical fiber amplifier doped with rare-earth elements (hereinafter referred to as a xe2x80x9crare-earth-doped fiber amplifierxe2x80x9d) 101, optical filter 102, polarization controller 103, optical splitter 104, optical modulator 105, optical isolator 106, Fabry-Perot filter 107, electrical (RF) oscillator 108, amplifier 109 and temperature controller 110.
The rare-earth-doped fiber amplifier 101 consists mainly of an optical fiber doped with a rare-earth element, a pump source which excites the fiber, an optical coupler and an optical isolator. This amplifier is connected in the form of a loop via the optical filter, polarization controller, optical modulator, optical isolator and Fabry-Perot filter, thereby forming a laser resonator 100.
The aforementioned higher-order mode-locked laser is characterized in that, in contrast to an ordinary mode-locked laser, a Fabry-Perot filter is installed within the resonator, and the modulation frequencies and characteristics of the Fabry-Perot filter or particularly the free spectral range (FSR) are set such that the excitation modes of the optical spectrum overlap with specific ones of the periodic pass spectra of the filter. More specifically, the conditions for performing higher-order (K-order) mode locking can be expressed by the following Equation 2.
Kxc2x7fm=Qxc2x7FSR=foxe2x80x83xe2x80x83(2)
Here, Q and K are positive integers with a mutually prime relationship with respect to each other and thus have no common prime factors. For example, Q=1, K=4; or Q=2, K=5. In Equation 2, fm is the modulation frequency and fo is the pulse repetition rate.
Also in order to perform higher-order mode locking, as in Equation 1, the modulation frequency must be selected as an integral multiple of the fundamental repetition rate fr of the resonator. Moreover, if the relationship between the FSR of the Fabry-Perot filter and the modulation frequency is as in Equation 2, a mode-locked pulse train is generated at a repetition rate K times the modulation frequency fm (fo=Kxc2x7fm=Qxc2x7FSR).
By using a Fabry-Perot filter with an FSR having a relationship with the modulation frequency such as that in Equation 2, in contrast to the case of ordinary mode locking wherein mode locking is applied by means of first-order modulation sidebands of modulation, in a higher-order mode-locked laser, K-order modulation sidebands are involved in mode locking. As a result, the repetition rate of pulses can be made to be K times the modulation frequency. Reference 3 above reports a technique whereby phase modulation is performed at 16 GHz and a Fabry-Perot filter with an FSR of 64 GHz is used (Q=1, K=4) to generate a pulse train with a repetition rate of 64 GHz. As another example, Reference 2 above reports an example wherein phase modulation is performed at a frequency of 5.79 GHz and a Fabry-Perot filter with an FSR of 3.48 GHz is used to generate a pulse train with a repetition rate of 17.4 GHz. In this case, Q=5 and K=3.
As described above, by performing higher-order mode locking, it is possible to generate pulse trains with a high repetition rate that was not possible with ordinary mode locking. For example, if fourth-order mode locking is performed using a 40-GHz phase modulator which has the broadest bandwidth commercially available and a Fabry-Perot filter with an FSR of 160 GHz, it is expected that a pulse train can be generated with a repetition rate of 160 GHz.
However, with a conventional higher-mode mode-locked laser pulse generator as described above, there are problems in that when pulses are generated over a long period of time, the length of the optical path of the resonator changes due to expansion or changes in the optical characteristics due to changes in the temperature of the constituent members, or when used in a vibrating environment, the optical path length changes due to vibration of components, and thus changes in the fundamental repetition rate fr occur and so the conditions for mode locking given in Equation 1 are not satisfied over long periods of time. To wit, with a conventional higher-order mode-locked laser pulse generator, the optical path length of the resonator changes particularly due to increases in the temperature of the optical fiber, so divergence occurs between the external modulation frequency and the fundamental repetition rate and thus mode locking is not easily achieved. As a result, there is a problem in that the width of pulses in the laser resonator and the spectral characteristics vary over time.
Here follows a description of just how much divergence occurs in the harmonic frequencies between the external modulation frequency and the fundamental repetition rate when the temperature of the interior of the laser resonator varies by xcex94t. Taking the change in the length of the optical fiber to be xcex94L when the temperature change is xcex94t, while xcex94t and xcex94L are both minute, they can be assumed to have the following proportional relationship.                                           Δ            ⁢                          xe2x80x83                        ⁢            L                    L                =                              α            ·            Δ                    ⁢                      xe2x80x83                    ⁢          t                                    (        3        )            
Here, L is the optical path length of the laser resonator prior to the temperature change and xcex1 is the coefficient of linear thermal expansion of the optical fiber.
Assuming the modulation frequency prior to the temperature change to be fm (=Nxc2x7fr), the divergence xcex94f between the modulation frequency in optimal mode locking and the actual modulation frequency is as follows.                               Δ          ⁢                      xe2x80x83                    ⁢          f                =                                            f              m                        ·                                          Δ                ⁢                                  xe2x80x83                                ⁢                L                            L                                =                                                    f                m                            ·              α              ·              Δ                        ⁢                          xe2x80x83                        ⁢            t                                              (        4        )            
If the temperature of the interior of the resonator varies by 0.1xc2x0 C. for example, assuming L=50 m, fm=40 MHz and xcex1=10xe2x88x925, based on Equation 3 and Equation 4, this gives xcex94L=50 xcexcm and xcex94f=40 kHz.
In this manner, an increase in the temperature of the resonator causes changes in the optical path length, so the fundamental repetition rate of the laser changes. In order to prevent this, known methods of stabilizing an ordinary mode-locking laser (in this case, the pulse repetition rate fo=modulation frequency fm) include: 1) the pulse phase locking method and 2) the method of performing stabilization using a Fabry-Perot filter. These are described below.
1) Pulse Phase Locking:
Shan, et al. reported in Reference 4 (Shan, et al., xe2x80x9cStabilizing Er fiber soliton laser with pulse phase locking,xe2x80x9d X. Electronics Letters, Vol. 28, No. 2, pp. 182-184, 1992) regarding a method of stabilizing a mode-locked laser.
FIG. 2 shows a laser apparatus based on this stabilization method. In the apparatus of FIG. 2, an erbium-doped optical fiber 201, modulator 205, polarization controller 203, optical splitter 204 and optical isolator 206 are connected in a ring using optical fiber, thus forming a resonator. In addition, in order to generate the optical soliton effect, an optical fiber with a stepped index of refraction (step index fiber) 202 is installed. In addition, the erbium-doped optical fiber is wound around a piezoelectric transducer (PZT) 214, and this piezoelectric transducer (PZT) 214 constitutes a portion of a feedback circuit for performing stabilization.
In this feedback circuit, a portion of the output laser pulse light is provided as input to an optical detector 215, and the electrical pulse signal thus obtained is amplified using an amplifier 211 and passed through a narrow bandpass filter 212. Thus, sinusoidal electrical signal components with the same frequency as the modulation frequency are extracted. Moreover, the phase difference between these sinusoidal signals and the electrical signals that drive the modulator is detected using a mixer 213, and this phase-difference signal is used as an error signal for the feedback circuit. This signal is amplified with a high-voltage amplifier and applied to the PZT which exhibits the piezoelectric effect, thereby deforming the PZT. To wit, by adjusting the voltage applied to the PZT, the length of the erbium-doped optical fiber wrapped around the PZT is adjusted, thereby applying compensation which cancels out the change in the optical path length of the resonator due to the temperature change. Since the optical path length of the laser resonator can be kept constant in this manner, stable operation is possible.
The aforementioned method is extremely effective in the stabilization of a mode-locked laser, but in order to implement this method, as is evident from the aforementioned description, the optical detector 215, amplifier 211, narrow bandpass filter 212 and mixer 213 must operate within the bandwidth of the pulse repetition rate. In particular, optical detection becomes difficult in the region wherein the pulse repetition rate is 100 GHz or greater, and moreover, the amplifier, bandpass filter and mixer are expensive, thus leading to increased manufacturing costs.
Moreover, when higher-order mode locking is performed, since the repetition rate of the pulses generated by the laser and the frequency of signals from the oscillator are not the same (K versus 1), additional effort is required for the generation of the error signal so it is clear that it cannot be applied to a higher-order mode-locked laser as is like above.
2) Method of Performing Stabilization Using a Fabry-Perot Filter
A stabilization method for performing the stabilization of an ordinary harmonic mode-locked laser (in this case, the pulse repetition rate fo=modulation frequency fm) without requiring high-bandwidth detectors, amplifiers, bandpass filters or other RF elements is recited by George T. Harvey in Reference 5 (Japanese Patent No. 2724278) and Reference 6 (U.S. Pat. No. 5,274,659).
FIG. 3 shows the constitution of such a laser apparatus. This laser consists of an erbium amplifier 15, optical couplers 13 and 27, pump laser 14, isolator 28, modulator 18, polarization controller 17, electronic oscillator 19, Fabry-Perot resonator 24, length adjustment apparatus 32 and a single-mode optical fiber 12 connected in a loop.
In the length adjustment apparatus 32, beam 33 and beam 34 are derived from the laser optical path. Next, these beams 33 and 34 are directed through a single wedged etalon 37, and disposed such that they pass through a first or second filter constituted differently depending on the location used. In addition, a beam 35 which constitutes part of the ring laser resonator is constituted such that it passes through a portion of the etalon intermediate to those where beam 33 and beam 34 pass. A differential amplifier 42 detects the difference in the light intensity detected by detectors 39 and 40 and an optical path length adjustment device 43 adjusts the length of the ring laser optical path, thereby compensating for fluctuations in the length of the optical path due to changes in the laser temperature.
In the aforementioned apparatus, the Fabry-Perot resonator 24 is detuned or deviated slightly from its frequency determined by the modulation frequency for the purpose of permitting an error signal to be generated that can be used to compensate for small changes in the length of the optical path of the closed-loop ring of the laser. In FIG. 4(a), the Fabry-Perot resonator has been detuned such that the FSR deviates from a frequency exactly equal to the pulse repetition rate by a frequency equal to df. This amount of detuning df required to perform stabilization is smaller than the fundamental repetition rate fn or namely df less than fn and this is one of the characteristics of this method. By slightly detuning the Fabry-Perot resonator in this manner, a small change in the length of the ring can be detected as an electrical signal through changes in the wavelength or frequency of output light, as described below.
As shown in FIG. 4(b), if a small change in the length of the optical path causes the ring mode M2 of Fabry-Perot mode R4 determined by the length of the optical path of the closed-loop ring of the laser to move slightly to the right, the transmission intensity at that frequency is greatly reduced, whereas the same change moves the corresponding ring mode M2 of Fabry-Perot mode R3 to a region of maximum output. The consequence of this is that the small change results in a large increase in the intensity of frequencies defined by mode R3 and a reduction of those in mode R5. In addition, a change in ring length that would have caused M2 to move in the opposite direction will cause a predominant shift of frequencies to those defined by Fabry-Perot resonant-mode R5. Thus, movements of M2 within resonator R4 to the right cause the light intensity in mode R3 to increase, and movements of M2 within R4 to the left cause the intensity of light in R5 to increase relative to the other Fabry-Perot modes R. Small changes in optical path length are thereby manifested as detectable changes in output frequency, thereby changing the spectral intensity distribution. This spectral intensity distribution can be detected as follows.
FIG. 3 shows apparatus 32 for detecting a change of frequency of the light transmitted in the ring laser resonator and automatically making the adjustment of the optical path length of the ring in response to such frequency deviation. Beam splitters are used for deriving from the optical path two optical beams 33 and 34. Here, a beam portion 35 constitutes part of the optical path of the ring. The three beam portions 33, 34 and 35 all constitute the optical path including the wedged etalon 37. This etalon 37 may be a body of quartz tapered as shown. Here, beam 33 is transmitted through a relatively thinner portion of the etalon, while beam 34 is transmitted through a relatively thicker portion.
Next, referring to FIG. 5, we shall describe the effects of this apparatus 32. FIG. 5 shows differences in characteristics depending on the position of the wedged etalon. This etalon constitutes an optical filter for each of the three beams. These characteristics, as shown in FIG. 5, are curve 55 representing the optical pass-band for beam 35, curve 54 representing the pass-band for optical beam 34, and curve 53 being the pass-band for beam 33. The intersection C of pass-bands 54 and 53 at frequency fc indicates that they are symmetrical with respect to fc. In addition, the intensities of beams 33 and 34 are detected by detectors 39 and 40, and since pass-bands 55 and 54 are frequency dependent, changes in optical frequency are manifested by changes of optical intensity detected by detectors 39 and 40. The outputs of the detectors are input to a differential amplifier 42 and the differential amplifier 42 output is input to an optical path length adjustment device 43. As described above, the length adjustment device 43 adjusts the length of the optical path of the ring laser based on the signals that detect changes in the optical path length due to temperature changes or the like, such that it is equivalent to being constant.
From the foregoing, it can be appreciated that slight detuning the Fabry-Perot resonator 24 of FIG. 3 can be quite effective in generating a difference voltage from differential amplifier 42 as needed for compensating for length changes. That is, as shown in FIGS. 4(a) and 4(b), a small change in the length of the optical path gives a large change in the intensity of resonator modes R3 and R5, and this large change in intensity in turn is converted to an electrical signal which is amplified by the difference amplifier, thus driving the optical path length controller. Here, in the absence of the detuning depicted in FIGS. 4(a) and 4(b), the change in the length of the optical path of the ring resonator could not be expected to generate a difference signal for making a length adjustment.
A drawback of this method is the problem that, if the bandwidth BW of the Fabry-Perot filter becomes much larger than the fundamental repetition rate fr of the laser, namely in the case of BW greater than  greater than fn it does not work well. The reason for this is described below.
In the case of BW greater than  greater than fn the relationship between the Fabry-Perot modes Rn and the oscillator modes and oscillator vertical modes may be illustrated in FIG. 15, for example. FIG. 15 shows an example wherein the bandwidth BW=10xc3x97fr and FSR=fmxe2x88x92df. While mode M2 indicated by the solid line is the generated mode, the modes indicated by dashed lines indicate the positions of vertical modes near M2 at which generation is suppressed. According to the conventional Fabry-Perot based stabilization method described in Reference 5 above, the amount of detuning df for generating the error signal must be selected to be a value smaller than fr (df less than fr). However, the case shown in FIG. 15, even if the mode M2 moves to the right or left due to temperature changes, the amount of this movement is smaller than the bandwidth BW, so as shown in FIG. 4, it is difficult to obtain as a fluctuation in intensity of a level that can be detected.
In the preferred embodiment of Reference 5, the frequency separation between modes of the resonator is fr=7 MHz, the width of each Fabry-Perot mode R is 16 MHz, the FSR is approximately 2.5 GHz, and the detuning frequency df is roughly 100 kHz. Therefore, the relationship between the filter bandwidth BW and the fundamental repetition rate fr is BW/fr=2.3, so it can be seen that this is an extremely small value. In addition, since FSR is roughly the same as the modulation frequency, the finesse of the filter (=FSR/BW) is a value close to fm/BW. If the modulation frequency is raised (for example, when fm=40 GHz), the value of the finesse required in the aforementioned laser structure becomes 2500. The manufacture and use of a Fabry-Perot filter having such a high finesse is difficult in practice.
Moreover, in order for the apparatus to operate stably, fm must be set in the range FSRxe2x88x92fr less than fm less than FSR+fn but there is a problem in that the modulation frequency or pulse repetition rate cannot be changed by 2xc3x97fr or greater.
Conventional stabilized mode-locked lasers have the following problems.
In the pulse phase locking method 1) above, the optical detector 215, amplifier 211, narrow bandpass filter 212 and mixer 213 included in the feedback circuit must operate within the bandwidth of the pulse repetition rate. However, optical detection becomes difficult when the pulse repetition rate is 100 GHz or greater, and moreover, the amplifier, bandpass filter and mixer are expensive, thus increasing manufacturing costs.
Moreover, when higher-order mode locking is performed with this method, since the repetition rate of the pulses generated by the laser and the frequency of signals from the oscillator are not the same (K versus 1), additional effort is required for the generation of the error signal so it is clear that it cannot be applied to a higher-order mode-locked laser as is like above.
In the method of performing stabilization using a Fabry-Perot filter 2) above, there is the problem in that if the bandwidth BW of the Fabry-Perot filter becomes much larger than the fundamental repetition rate fr of the laser (BW greater than  greater than fr) it does not work well. In order to generate pulses stably at a repetition rate of several dozen GHz or 100 GHz or greater, the BW of the Fabry-Perot filter must be selected at roughly fr and one with an extremely large finesse is required, so there are problems with manufacture and use, and the cost also becomes high.
Moreover, an additional drawback of this method is the problem that it is difficult to apply to lasers that oscillate at a plurality of modes with adjacent frequencies. This is because when the signal serving as the basis is positioned between a plurality of adjacent modes, a feedback signal cannot be obtained or is difficult to obtain.
The present invention has come about in consideration of the above problems. One object thereof is to provide a mode-locked laser apparatus that, while being a higher-order mode-locked laser that is able to generate pulses with a high repetition rate, is also able to perform frequency-stabilized pulse generation over long periods of time without using wide-bandwidth components.
In order to achieve the aforementioned object, the present invention provides a mode-locked laser apparatus comprising: a mode-locked laser oscillator, detection means for detecting changes in length of an optical path of the mode-locked laser oscillator by utilizing chromatic dispersion characteristics of the optical path, an optical path length controller that controls the length of the optical path of the laser oscillator, and a feedback circuit that controls the optical path length controller by means of a signal detected by the detection means.
The invention further provides a mode-locked laser apparatus comprising: a mode-locked laser oscillator, an optical modulator, a signal generator that moves the optical modulator with a modulation signal with a constant frequency, an optical filter, an optical isolator, detection means for detecting changes in length of an optical path of the mode-locked laser oscillator by utilizing chromatic dispersion characteristics of the optical path, an amplifier that amplifies an output signal obtained from the detection means, an optical path length controller that controls the length of the optical path of the laser oscillator, and a feedback circuit that controls the optical path length controller by means of the output signal amplified by the amplifier.
The invention additionally provides a mode-locked laser apparatus comprising: a mode-locked laser oscillator, an optical modulator that modulates light output from the laser oscillator with a modulation signal, filter means that selects sidebands equivalent to harmonics of the modulation signal contained in the light modulated by the optical modulator, an optical isolator, detection means for detecting changes in length of an optical path of the mode-locked laser oscillator by utilizing chromatic dispersion characteristics of the optical path, an amplifier that amplifies an output signal obtained from the detection means, an optical path length controller that controls the length of the optical path of the laser oscillator, and a feedback circuit that controls the optical path length controller by means of the output signal amplified by the amplifier.
The mode-locked laser apparatus just mentioned above, wherein a modulation frequency of the optical modulator and a free spectral range of the filter means have relationship of being equal to each other when one of the modulation frequency and the free spectral range is multiplied by one of two mutually prime positive integers K and Q and the other multiplied by the other integer, and wherein the filter means has a combination in which the positive integers K and Q are present such that a ratio K/Q is a value below a finesse of a Fabry-Perot filter, can further comprises: a first construction for extracting from the output light two spectral frequency components at roughly equal distances in frequency from a center frequency of an oscillation spectrum of the output light, one on a long-frequency side and the other on a short-frequency side, a second construction for using two photodetectors to detect an average intensity of each of the two frequency components extracted by the first construction, means of deriving an intensity-difference signal for two signals detected by the second construction, and a construction for controlling the optical path length controller in accordance with the intensity-difference signal, and wherein a pulse train is generated at a repetition rate that is the same as the modulation frequency.
The invention additionally provides a mode-locked laser apparatus comprising: a mode-locked laser oscillator, an optical modulator that modulates light output from the laser oscillator with a modulation signal, detection means for detecting changes in length of an optical path of the mode-locked laser oscillator by utilizing chromatic dispersion characteristics of the optical path, an amplifier that amplifies an output signal obtained from the detection means, an optical path length controller that controls the length of the optical path of the laser oscillator, and a feedback circuit that controls the optical path length controller by means of the output signal amplified by the amplifier.
The mode-locked laser apparatus just mentioned above can further comprises: a first construction for extracting from the output light two spectral frequency components at roughly equal distances in frequency from a center frequency of an oscillation spectrum of the output light, one on a long-frequency side and the other on a short-frequency side, a second construction for using two photodetectors to detect an average intensity of each of the two frequency components extracted by the first construction, means of deriving an intensity-difference signal the two signals detected by the second construction, and a third construction for controlling the optical path length controller in accordance with the intensity-difference signal, and wherein a pulse train is generated at a repetition rate that is the same as the modulation frequency.
Any one of the aforementioned mode-locked laser apparatus can further comprise dispersion control means included in the optical path, dispersion characteristics of which are to be detected.
In the mode-locked laser apparatus just mentioned above, the dispersion control means is a section of an optical fiber with appropriate dispersion and length.
In the mode-locked laser apparatus just mentioned above, the dispersion control means can be a chirped fiber Bragg grating.
In any one of the fourth and sixth to tenth mentioned mode-locked laser apparatus, the first construction can consist of two bandpass filters.
In the mode-locked laser apparatus just mentioned above, the two bandpass filters each have variable filter characteristics, and comprise means of varying their respective filter characteristics before or during the use of the mode-locked laser apparatus in order to maintain a predetermined relationship among the filter characteristics.
As described above, with the mode-locked laser apparatus according to the present invention, it is possible to use the chromatic dispersion characteristics of the optical path to generate a feedback signal for the optical path length. So, it is possible to use an even wider-bandwidth Fabry-Perot filter without requiring use of electrical devices with wide bandwidth characteristics, and thus the manufacturing cost can be reduced.
In addition, with the present invention, in a higher-order mode-locked laser, it is possible to utilize the chromatic dispersion characteristics of the optical path to detect the optical path length and generate a feedback signal for the optical path length.
Moreover, with the present invention, it is possible to utilize the chromatic dispersion characteristics of the optical path to generate a feedback signal for the optical path length. So, pulses with a high repetition rate can be generated readily with a higher-order mode-locked laser, and thus detection of chromatic dispersion characteristics can be done readily in this region also.
The above and other objects and features of the invention will be better understood from a consideration of the following detailed description based upon the accompanying drawing.